The present invention relates to a clamp for an injection molding machine which uses an electric screw drive for clamp stroke motions and at least one column engaging a hydraulic clamp piston to provide clamp force. The present invention also includes a self adjusting mechanism connected to the column so that dimensional changes caused by clamp deflections do not damage the drive means.
The prior art includes many injection molding machines which use tiebar clamping units for positioning a platen for the application of clamp-up forces. Control systems using hydraulic fluid, pressurized air and electric motors in combination with numerous switches and typically a controller unit are used to control both the positioning of the platens and the application and removal of clamp-up force for acquiring mold clamp-up and mold break.
Japanese Patent 61261-017 discloses a clamping mechanism for injection molding machines. The purpose of the control system therein is to maintain a parallel parting line between the molds by sensing the distance traveled by each clamping cylinder of each tiebar and maintaining the distances substantially equal. Accordingly, parallelism of the parting line between the molds can be maintained when the injection pressure of molten resin is uneven within the mold.
U.S. Pat. No. 4,832,884 to Speck et al. discloses a method for measuring and controlling the closing force of a plastic injection molding machine. Accordingly, for an injection molding machine, a closing force is measured for a predetermined number of operation cycles. A mean value is calculated from the measurements and if the mean value is within a predetermined tolerance, no control intervention takes place. However, if the mean value is outside the tolerance but within a zone limited by alarm units, control intervention takes place by stepwise changing the installed height of the mold until the actual value measured for closing force after each operating cycle is within the tolerance given. In this method and system, a toggle joint system serves to generate the closing force on the first and second mold parts. In this case, the force is measured and if the force is within a tolerance zone, the spacing between the first and second mold parts is altered to less than or no increase relative the old force.
U.S. Pat. No. 4,966,738 to Inaba et al. discloses a method for mold clamping force adjustment. Accordingly, in this method, a mold touch position is detected, at which the mold halves of a mold contact each other. The mold temperature is detected by means of thermocouple attached to the mold halves. The mold thickness is obtained based on the position where a movable platen and a stationary platen of the injection molding machine contact each other. After a molding operation is started, the mold temperature is detected and the amount of change in mold thickness during a period between a preceding cycle and a current cycle is calculated based on the change of mold temperature, the mold thickness, and the thermal expansion coefficient of the mold. A mold touch position for a current cycle is obtained based on the calculated value. Accordingly, when the mold is clamped in the current cycle, the movable platen is moved from the mold touch position toward the stationary platen by a predetermined amount to produce a predetermined mold clamping force. For this method, the measurements and adjustments are directed primarily to the mold position and not to the position of the clamping units. A clamping unit is a singular one which functions to force the movable platen against the stationary platen. That is, no tiebar clamping units are used internal to any of the platens for producing the clamp-up force.
U.S. Pat. No. 5,147,661 to Kurumaji et al. discloses a mold aligning device for a compression molding machine. The mold aligning device includes a plurality of mold position adjusting cylinder actuators disposed on a bed wherein position detectors are associated with the mold positioning adjusting cylinder actuators to detect the strokes thereof. The hydraulic source for driving the mold position adjusting cylinder actuators and control unit for controlling the strokes of the piston rods of the actuators with reference to zero points of the piston rods of the actuators, is determined beforehand. The zero points are decided by placing the upper mold in close contact with the lower mold and extending the piston rods of the actuators so that the piston rods are pressed against the lower surface of a slide block. The strokes of the piston rods are controlled during a compression-molding operation so that the upper mold is maintained precisely in parallel with the lower mold. This device is directed to pressure molding. The cylinder actuators are not positioned within a movable molten platen, and the actuators do not clamp onto tiebars. The main purpose of the machine is to maintain parallelism between the upper and lower molds by the actuators placing direct pressure on the upper mold supporting surface for maintaining the same parallel to the lower mold.
U.S. Pat. No. 5,338,171 to Hayakawa et al. discloses a die-clamping apparatus with an aligning device. The apparatus includes a stationary die plate for holding a stationary die, a movable die plate for holding a movable die, a hydraulic cylinder for moving the movable die plate forwardly and rearwardly with respect to the stationary die plate, and a tiebar for clamping by a fastening device located in the movable die plate. One or more alignment devices are provided so as to join the movable die and the stationary die in such a manner that primary alignment can be performed. The molding apparatus can be provided with a guide pin for secondarily aligning the movable die with respect to the stationary die when the stationary die and the movable die have been placed at predetermined positions. Four die clamping cylinders are provided at the corners of the stationary die plate so as to apply a clamping force to the stationary and movable dies after the fastening device has fastened to the tiebar. For this device, the tiebar fastener and clamping units, while being located on the tiebars, one adjacent the movable platen and one adjacent the stationary platen, are separate devices adding to the complexity of the machine.
U.S. Pat. No. 5,370,518 to Sasaki et al. discloses an apparatus for injection and compression molding. The apparatus includes an injection device and a compression molding device wherein the compression molding device includes a stationary die plate for holding a stationary die, a movable die plate for holding a movable die, means for rapidly extending and retracting the movable die plate relative to the stationary die plate and means for locking the movable die plate to the tiebars at a position where the movable die plate approaches the stationary die plate. Means for fastening the dies after the movable die plate is locked to the tiebars is also provided. An injection device for feeding a molten plastic material into a mold cavity between the stationary die and the movable die initiates feeding of the molten plastic material into the mold cavity when the stationary die and movable die are parted from each other by a predetermined distance. The plastic material is compressed and drawn while the movable die is moved towards the stationary die after a predetermined quantity of molten plastic material has been fed into the mold cavity. The apparatus includes a device in a movable die plate for clamping onto the tiebars. However, the device does not include a combined means for providing the clamping force between the platens and fastening to the tiebars, as the clamp force is provided by separate die fastening cylinders which are positioned on the tiebars but separate from the fasteners.
U.S. Pat. No. 5,133,655 to Schad et al. shows a clamp mechanism for an injection molding machine in which four columns attached to the moving platen are individually gripped by fluid actuated cylinders which also act as clamp force pistons. While this design is effective, it requires very close tolerances to be maintained between the columns and the inner bores of the cylinders since the amount of deflection of the cylinder walls to achieve sufficient gripping on the columns is small. Thus, manufacturing this clamp is expensive and in operation maintaining these small clearances imposes costly maintenance procedures.
U.S. Pat. Nos. 5,620,723, 5,624,695 and 5,645,875 to Glaesener show a novel lock nut arrangement for engaging the tiebars of a two platen injection molding machine clamp, for example. The locking nuts also act as clamp force pistons acting on the tiebars to clamp the mold between the platens.
The combination of a screw driven clamp stroke and hydraulic clamping are shown in U.S. Pat. No. 3,191,235 to Rougement, Japanese Patent 2-252518, and European Patent Publication 0 508 277 to Miese et al. In the '235 and '277 references, a single clamp piston is carried inside a cylinder in the moving platen. The rod of the clamp piston is directly connected to a single column which is threaded along most of its length such that a nut can engage the thread and either the column or nut are rotated to cause the column, clamp piston and moving platen to move. Once the screw drive has closed the mold, high pressure hydraulic oil is supplied to the non-rod side of the clamp piston to cause the moving platen to exert a clamping pressure against the mold.
The '518 reference teaches a similar threaded column for mold closing in combination with a variety of means for exerting clamp force against the mold. FIGS. 1-3 in the '518 reference illustrate the invention, which is a column blocking method. Center column 46 is attached to the moving platen 5, which in turn is moved by nuts 32 engaging threaded columns 31 to cause stroke motion. When the moving platen has moved to the mold closed position (see FIGS. 1 and 2), shutter 49 is moved by cylinder 50 between the end of the column 46 and sleeve 39 blocking column movement. Sleeve 39 is threaded into shutheight adjustment nut 38 carried in the clamp block 30. By rotating the nut 38, the end of the sleeve 39 that engages the shutter can be moved with respect to the block 30, thereby adjusting the distance between the closed platens 5 and 3 to provide for molds having different shutheights. Clamping force is generated by hydraulic cylinders 35 acting on the ends of tiebars 4 and mounted in the clamp block 35.
U.S. Pat. No. 4,645,443 to Aoki shows a hydraulic clamp having both stroke and clamp functions performed by hydraulic cylinders acting through a single column. The shutheight adjustment is made using a threaded connection between the column end and the moving platen. Thus, clamping forces are transmitted via the column through the threaded connection to the moving platen.
PCT Publication US97/19507, published May 14, 1998, shows a means for applying a clamping force to a movable platen. At least one column is affixed to the movable platen. A plurality of circumferentially spaced teeth is provided on the end of the column spaced from the movable platen, and a clamp piston and lock means engaging the clamp piston are provided operative to engage and disengage the teeth.
It is highly desirable to provide a means for accommodating the variations in column length that can occur due to the forces generated by the clamping piston acting on the column. This is particularly important when the column transmits the clamping and the opposed mold break forces to the moving platen, as described in the above PCT publication.
Accordingly, it is an object of the present invention to provide a fast acting and energy efficient injection molding machine clamp assembly and clamping method.
It is a still further object of the present invention to provide an improved injection molding machine clamp assembly and method as aforesaid with low construction cost, low maintenance and good reliability.
It is a still further object of the present invention to provide an improved injection molding machine clamp assembly and method which occupies less floor space than conventional designs.
It is an additional object of the present invention to provide an improved injection molding machine clamp assembly and method which accommodates variations in column length that can occur due to the forces generated by the clamping piston acting on the piston.
Further objects and advantages of the present invention will appear hereinbelow.